I think the simulation of sieges isn't correct either, at the moment. The AI can summon up huge armies of 600-1200 men and overwhelm a castle or town garrison of 200-400 men with ease. If there isn't a multiplier advantage for being behind fortifications, there should be! If a town has 150 garrison soldiers and 250-300 militia, they should be able to hold off an attacking force of twice their size since they will have the interior lines for movement, be fighting from behind walls and able to enfilade attackers with archery and war machines.